The problem of error correction in memories of conventional design has been already addressed heretofore.
U.S. Pat. No. 4,958,352, for example, discloses a semiconductor memory provided with an ECC (Error Check and Correcting) circuit. The memory comprises a first cell matrix for the principal information, and a second cell matrix for the ECC information. Associated with each information word, an 8-bit word in the example, is an error check and correcting word, a 4-bit word in the example, which only allows a single error to be located and corrected (this being generally found adequate). Each bit of binary data is stored in a different one of the memory cells.
Understandably, by providing memory cells which can store, for example, four bits of data, it becomes possible to store, in the instance of the above example, each information word into two cells of the first array and the corresponding error check and correcting word into a cell of the second array.
Using Hamming codes, as the codes which provide single-error check and correcting words of shortest length, the relation 2.sup.M =N+1+M applies, where N is the bit length of the information word, and M is the bit length of the ECC word. There are, of course, other code types which allow a larger number of errors to be checked and/or corrected, but they require longer error check and correcting words. The subject is exhaustively dealt with in Jiri Adamek, "Foundations of Coding", John Wiley & Sons, 1991.
Multi-level memories are currently the subject of intensive research work because of the growing demand for semiconductor storage devices of large capacity, in particular non-volatile ones. In memories of this kind, write and/or read errors are much more likely to occur, and steps must be taken to prevent or at least reduce them.
In investigating the operation of multi-level memories, in particular of the EEPROM and Flash EPROM types, it has been found that when a cell is programmed improperly, more than one data bit in that cell are incorrect.
Of course, it would be possible to use a code which allows a large number of errors to be checked and corrected, but this results in the error corrective array attaining impractical dimensions, well in excess of those of the informational matrix.